MC68HC908GT16 Datasheet, PDF (118/412 Pages) Motorola, Inc – Microcontrollers

Internal Clock Generator (ICG) Module
The amplifier is enabled when the external clock generator enable
(ECGEN) signal is set and when the external crystal enable
(EXTXTALEN) bit in the CONFIG2 register is set. ECGEN is controlled
by the clock enable circuit (see 7.4.1 Clock Enable Circuit) and
indicates that the external clock function is desired. When enabled, the
amplifier will be connected between the PTE4/OSC1 and PTE3/OSC2
pins. Otherwise, the PTE3/OSC2 pin reverts to its port function.
In its typical configuration, the external oscillator requires five external
components:
1. Crystal, X₁
2. Fixed capacitor, C₁
3. Tuning capacitor, C₂ (can also be a fixed capacitor)
4. Feedback resistor, RB
5. Series resistor, R_S (Included in Figure 7-3 to follow strict Pierce
oscillator guidelines and may not be required for all ranges of
operation, especially with high frequency crystals. Refer to the
crystal manufacturer’s data for more information.)
7.4.3.2 External Clock Input Path
The external clock input path is the means by which the microcontroller
uses an external clock source. The input to the path is the PTE4/OSC1
pin and the output is the external clock (ECLK). The path, which contains
input buffering, is enabled when the external clock generator enable
signal (ECGEN) is set. When not enabled, the PTE4/OSC1 pin reverts
to its port function.
Technical Data
118
MC68HC908GT16 • MC68HC908GT8 — Rev. 2
Internal Clock Generator (ICG) Module
MOTOROLA